[travis] Install Cabal library
[travis] Install Cabal library

- CI builds using GHC 7.6.3 will fail since appropriate Cabal library is missing
- This change makes sure to install Cabal
- Closes https://github.com/Microsoft/bond/issues/384
tstein/bond,chwarr/bond,jdubrule/bond,Microsoft/bond,sapek/bond,tstein/bond,chwarr/bond,Microsoft/bond,tstein/bond,gencer/bond,Microsoft/bond,chwarr/bond,jdubrule/bond,tstein/bond,chwarr/bond,gencer/bond,tstein/bond,jdubrule/bond,gencer/bond,Microsoft/bond,sapek/bond,gencer/bond,jdubrule/bond,sapek/bond,Microsoft/bond,gencer/bond,sapek/bond,sapek/bond,chwarr/bond,tstein/bond,chwarr/bond,Microsoft/bond,jdubrule/bond,jdubrule/bond,sapek/bond